It has been estimated that the global fuel cells market (valued by some analysts at US $355.3 million in 2011) is projected to grow to US $910.3 million by 2018, growing at a CAGR of 15.0% from 2013 to 2018. The fuel cells market is expected to grow at a compound annual growth rate of 15% during the forecast period. Among various types of fuel cells, proton exchange membrane fuel cells (PEMFCs) have the greatest potential in penetrating the market, especially in portable applications. The commonly used fuel for PEMFCs is hydrogen where it reacts with oxygen to produce electricity, heat and water. PEMFCs are viewed as leading the overall fuel cells market in terms of revenue, and estimated to be growing at a CAGR of 11.5% from 2013 to 2018 and accounting for 46.3% of the total demand in 2011 in terms of power volume (MW).
Hydrogen can also be used in a diesel engine system with no or little modifications to the engine. Hydrogen can be introduced into the engine either by carburation, manifold/port injection or in-cylinder injection. The literature on diesel pilot-ignited hydrogen combustion suggest that hydrogen substitution is a promising method of reducing undesired exhaust emissions, especially at high rates of hydrogen substitution. There may be significant savings of fuel consumption as well.
It can thus be seen that hydrogen is an important fuel source and its generation and delivery to hydrogen-consuming devices is critical to the successful use of such devices.
Metal borohydrides have been developed as viable hydrogen carriers (U.S. Pat. Nos. 2,461,662, 2,461,663, 2,534,553 and 2,964,378). During the 1990s, sodium borohydride attracted a tremendous amount of attention due to its chemical properties including non-flammability of sodium borohydride solutions, high hydrogen density (HD, 10.8 wt. %) and the high stability of its environmentally safe reaction by-products. U.S. Pat. No. 6,534,033 describes a hydrogen generation system wherein the hydrolysis of sodium borohydride has been successfully demonstrated. However, hydrogen generation using this system does not appear to be suitable for heavy-duty applications due to issues relating to the handling of water, catalyst reactivity/deactivation and the treatment of by-products. These issues have been further discussed in the published article: J. H. Wee, K. Y. Lee, and S. H. Kim, Sodium borohydride as the hydrogen supplier for proton exchange membrane fuel cell systems, Fuel Processing Technology, 87 (2006) 811-819.
Another hydrogen generation reactor for portable fuel cell system is described in U.S. Pat. No. 7,105,033. In this reactor, alkaline stabilized sodium borohydride solution is injected into a fixed bed reactor filled with a catalyst, to cause high-speed generation of hydrogen gas from the reaction NaBH4+(2+n)H2O=NaBO2.nH2O+4H2. Despite its several advantages, such as, relatively controllable start and stop of hydrogen generation, this method still suffers from many deficiencies, in particular, a low hydrogen production density, non-constant hydrogen flow rate, short life span of catalyst, low solubility of by-products (sodium borate). Furthermore, the catalysts used are typically noble metals (Pt, Pd, etc.), adding to the cost pressures of running such systems. The cost of operating the system can be significantly lowered if robust self-support cobalt oxide-based catalyst is used as an alternative catalyst to accelerate the hydrolysis reaction of sodium borohydride (see US 20150017084 A1).
Nevertheless, there still remains a need to develop a viable hydrogen generator that can be cost effectively coupled with hydrogen-consuming devices to provide hydrogen as a fuel to such devices.